1. Field of the Invention
The present invention relates to a switching power supply apparatus and more particularly to a switching power supply apparatus for varying output voltage in response to change of power dissipation of an electronic apparatus operated by the switching power supply apparatus.
2. Description of the Related Art
In general, a power supply apparatus for outputting direct current rectifies alternating current from the outside, and supplies one single primary voltage required by an electronic apparatus. In cases where the electronic apparatus requires a secondary voltage different from the primary voltage, the power supply drops or raises the primary voltage as needed. Examples of methods used for converting the primary voltage applied to the electronic apparatus into the secondary voltage include a linear converting method and a switching converting method.
The linear converting method is a method for forming the secondary voltage by mainly dividing the primary voltage by means of a passive element such as a resistor. However, since the passive element generates heat according to the voltage drop, it causes a 50% power loss as well as showing low stability.
The switching converting method obtains direct current by turning on and off the primary voltage and outputting the same in the form of a pulse, and by smoothing the pulse. This reduces the weak points of the linear converting method, namely, high power loss and heat generation. The switching conversion has more than 90% power efficiency and generates little heat.
FIG. 1 is a conceptual block diagram of a switching power supply apparatus of a related art. The switching power supply apparatus includes a pulse generating unit 10, a switch 20, an output unit 30.
The pulse generating unit 10 sets a duty ratio by capacitance of the capacitor 11 connected with a capacitance input terminal CT, and outputs a PWM (Pulse Width Modulation) signal according to the set duty ratio. The switch 20 performs turn on and turn off operations between an input voltage Vin and the output unit 30 in response to the pulse width modulation signal output from the pulse generating unit 10.
The output unit 30 smoothes the input voltage Vin that is in a form of a pulse defined by operation of the switch 20, and outputs the same. The procedure for smoothing the pulse form input voltage is described in more detail below.
If the pulse width modulation signal Vsw from the pulse generating unit 10 is ‘high,’ a gate terminal of the switch 20 consisting of an NMOS (N-channel Metal Oxide Semiconductor) transistor is turned on so that the input voltage Vin is applied to the output terminal Vout through an inductor 31. At this moment, a diode 32 is reverse-biased, remaining at a blocked off status.
In contrast, if the pulse width modulation signal output from the pulse generating unit 10 is ‘low’, the switch 20 is turned off, and the input voltage Vin supplied to the inductor 31 is blocked off, whereby reverse electromotive force is generated instantly at the inductor 31. Accordingly, the diode 32 is turned on by the reverse electromotive force generated from the inductor 31, and a current path formed by the inductor 31, the capacitor 33, and the diode 32 discharges accumulated current.
By the foregoing procedure, namely, the procedure where turn-on and turn-off of the switch 20 are repeated, a voltage given by the following formula 1 is applied to the output terminal Vout of the switching power supply apparatus.                     Vout        =                                            t              on                                      (                                                t                  on                                +                                  t                  off                                            )                                ⁢          Vin                                    [                  Formula          ⁢                                          ⁢          1                ]            
Here, ton equals a time period during which the switch 20 is turned on, and toff equals a time period during which the switch 20 is turned off. The time periods ton, toff for determining an output voltage of the output terminal Vout, are determined by the capacity of the capacitor 11 connected with the capacitance input terminal CT of the pulse generating unit 10. As shown in FIG. 1, the capacity of the capacitor 11 is fixed.
Therefore, when power dissipation of the electronic apparatus (not shown) connected with the output terminal Vout is increased, the output voltage of the output terminal Vout is reduced.
While a reduction of the output voltage could be inhibited by an increase of the capacities of the capacitor 33 and the inductor 31, the size of the electronic apparatus equipped with such switching power supply apparatus would also be correspondingly increased. Thus, it would be difficult to adopt such an approach in a portable electronic apparatus.